U.S. patent number 4,179,729 [Application Number 05/787,930] was granted by the patent office on 1979-12-18 for rotary electric machine and power conversion system using same.
This patent grant is currently assigned to The Charles Stark Draper Laboratory, Inc.. Invention is credited to David B. Eisenhaure, Kenneth Fertig, George A. Oberbeck, William E. Stanton.
United States Patent |
4,179,729 |
Stanton , et al. |
December 18, 1979 |
Rotary electric machine and power conversion system using same
Abstract
A power conversion system for converting between electrical
power at different frequencies, including a rotary electrical
machine including a rotor, a stator, first and second independently
controllable field windings, and at least one armature winding for
each phase; a switching circuit including a plurality of switching
devices and having first terminal means interconnected with the
armature winding which carries a high frequency signal established
by the machine, and a second terminal for interconnection with an
impedance establishing a lower frequency signal; the first field
control circuit for monitoring the machine to sense phase
difference between the optimum zero crossings and actual zero
crossings of the higher frequency signal for driving the first
field winding to adjust the phase of the higher frequency signal to
minimize the phase difference; a second field control circuit for
modulating the higher frequency signal carried by the armature
winding with the lower frequency signal and for monitoring the
second terminal means to sense amplitude difference between a
selected one of the voltage and current parameters of the lower
frequency signal and a reference level for driving the second field
winding to adjust that parameter towards the reference level; and a
switch firing circuit responsive to the machine voltage and one of
the voltage and current parameters of the lower frequency signal at
the second terminal means for selectively triggering to the on
state and self-commutating to the off state the switching devices
synchronously with the zero crossings of the higher frequency
signal for transferring power between the higher and lower
frequency signals through the switching circuit.
Inventors: |
Stanton; William E. (Newton,
MA), Eisenhaure; David B. (Hull, MA), Oberbeck; George
A. (Belmont, MA), Fertig; Kenneth (Sudbury, MA) |
Assignee: |
The Charles Stark Draper
Laboratory, Inc. (Cambridge, MA)
|
Family
ID: |
25142941 |
Appl.
No.: |
05/787,930 |
Filed: |
April 15, 1977 |
Current U.S.
Class: |
363/175; 318/150;
318/158; 322/4 |
Current CPC
Class: |
H02J
3/34 (20130101); H02K 11/048 (20130101); H02M
7/02 (20130101); H02M 5/32 (20130101); H02K
47/18 (20130101) |
Current International
Class: |
H02M
7/02 (20060101); H02K 47/00 (20060101); H02K
47/18 (20060101); H02J 3/34 (20060101); H02M
5/02 (20060101); H02M 5/32 (20060101); H02M
007/00 () |
Field of
Search: |
;322/4,20,32,63,66
;318/150,158 ;307/73 ;363/102,150,174,175 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Interim Report on Research Toward Improved Flywheel Suspension and
Energy Conversion Systems", D. Eisenhower, G. Oberbeck, S. O'Dea,
W. Stanton, The Charles Stark Draper Laboratory, Inc., Mar.
1976..
|
Primary Examiner: Shoop; William M.
Attorney, Agent or Firm: Iandiorio; Joseph S.
Claims
What is claimed is:
1. A rotary electrical machine for use in power conversion between
a higher frequency power signal and a lower frequency power signal
including: a rotor; a stator; an armature; and first and second
independently controllable field windings in spatial quadrature for
selectively shaping the back EMF waveform of the machine to control
the amplitude and phase relation of the voltage and current outputs
of the machine;
a first field control circuit responsive to the phase difference
between the voltage and current of the higher frequency signal
generated in the armature winding of the machine for driving said
first field winding to minimize said phase difference;
a second field control circuit for monitoring the amplitude
difference between a selected one of the voltage and current
parameters of said lower frequency signal and a reference level for
driving said second field winding to adjust that load signal
amplitude toward said reference level; and
switching means responsive to said higher frequency signal and said
lower frequency signal for selectively transferring power between
said higher and lower frequency signals.
2. A power conversion system for converting between electrical
power at different frequencies, comprising:
a rotary electrical machine including a rotor, a stator, first and
second independently controllable field windings, and at least one
armature winding for each phase;
a switching circuit including a plurality of switching devices and
having first terminal means interconnected with said armature
winding which carries a higher frequency signal established by said
machine and a second terminal means for interconnection with an
impedance establishing a lower frequency signal;
a first field control circuit for monitoring said machine to sense
phase difference between the optimum zero crossings and actual zero
crossings of said higher frequency signal for driving said first
field winding to adjust the phase of said higher frequency signal
to minimize said phase difference;
a second field control circuit for modulating said higher frequency
signal carried by said armature winding with said lower frequency
signal and for monitoring said second terminal means to sense
amplitude difference between a selected one of the voltage and
current parameters of said lower frequency signal and a reference
level for driving said second field winding to adjust the parameter
towards said reference level; and
a switch-firing circuit responsive to machine voltage and said
voltage and current parameter of said lower frequency signal at
said second terminal means for selectively triggering to the on
state and self-commutating to the off state said switching devices
synchronously with said zero crossings of said higher frequency
signal for transferring power between said higher and lower
frequency signals through said switching circuit.
3. The system of claim 2 in which said machine is a motor generator
which is capable of operating in a motor mode and generator mode
and whose rotor is adapted for connection to a mechanical load and
drive, said impedance is an electrical load and power source, and
said power transfer through said switching circuit occurs in the
generator mode by detecting said lower frequency signal from said
higher frequency signal and presenting said lower frequency signal
at said second terminal means and in the motor mode by chopping
said lower frequency signal and presenting it at said first
terminal means.
4. The system of claim 2 in which said machine is a motor, said
impedance is an electrical power source, and said power transfer
through said switching circuit occurs by chopping of said lower
frequency signal at the frequency of said higher frequency signal
and presenting it at said first terminal means.
5. The system of claim 2 in which said machine is a generator, said
impedance is an electrical load, and said power transfer through
said switching circuit occurs by detecting the lower frequency
signal from the higher frequency signal and presenting it at said
second terminal.
6. The system of claim 2 in which said impedance includes an
electrical power source and said second field control circuit
monitors the current parameter of said lower frequency signal at
said second terminal means.
7. The system of claim 2 in which said impedance includes an
electrical load and said second field control circuit monitors the
voltage parameter of said lower frequency signal at said second
terminal means.
8. The system of claim 2 in which said field windings are in
spatial quadrature.
9. The system of claim 2 in which said field windings are on said
rotor.
10. The system of claim 2 in which said field windings are on said
stator.
11. The system of claim 2 in which said armature winding is on said
stator.
12. The system of claim 2 in which said machine is single phase and
includes one armature winding and one pair of first and second
field windings.
13. The system of claim 2 in which said machine is n phase and
includes n armature windings and n pairs of first and second field
windings.
14. The system of claim 2 in which said switching circuit includes
one set of switching devices for each phase, each set including
four pairs of parallel connected, oppositely polarized,
semiconductor switches interconnected between two bus lines
extending between said first and second terminal means, one of said
pairs connected in series in each of said bus lines and the other
two of said pairs cross-connected between said bus lines.
15. The system of claim 14 in which said semiconductor devices are
SCR's.
16. The system of claim 2 in which said first field control circuit
includes a rotor position sensor for determining said machine
voltage optimum zero crossing, a zero cross over detector circuit
for detecting actual zero crossovers of said higher frequency
signal at said first terminal, and a phase-sensitive detector
responsive to said rotor position sensor and said zero crossover
detector circuit.
17. The system of claim 2 in which said second field control
circuit includes a comparator circuit for comparing said selected
parameter with said reference level.
18. The system of claim 2 in which said switch firing circuit
includes logic means responsive to said actual zero crossovers and
optimum zero crossovers of said higher frequency signal at said
first terminal and to the voltage and current parameters of said
lower frequency signal at said second terminal means for
sequentially actuating selected ones of said switching devices.
19. A power conversion system for converting between electrical
power at different frequencies, comprising:
a motor generator which is capable of operating in a motor mode and
a generator mode and whose rotor is adapted for connection to a
mechanical load and drive, said motor-generator including a rotor,
a stator, first and second independently controllable field
windings, and at least one armature winding for each phase;
a switching circuit including a plurality of switching devices and
having first terminal means interconnected with said armature
winding which carries a higher frequency signal established by said
machine and a second terminal means for interconnection with an
electrical load and power source;
a first field control circuit for monitoring said machine to sense
phase difference between the optimum zero crossings of said higher
frequency signal for driving said first field winding to adjust the
phase of said higher frequency signal to minimize said phase
difference;
a second field control circuit for modulating said higher frequency
signal carried by said armature winding with said lower frequency
signal and for monitoring said second terminal means to sense
amplitude difference between a selected one of the voltage and
current parameters of said lower frequency signal and a reference
level for driving said second field winding to adjust that
parameter towards said reference level; and a switch-firing circuit
responsive to machine voltage and said voltage and current
parameters of said lower frequency signal at said second terminal
means for selectively triggering to the on state and
self-commutating to the off state said switching devices
synchronously with said zero crossings of said higher frequency
signal for transferring power between said higher and lower
frequency signals through said switching circuit in the generator
mode by detecting said lower frequency signal from said higher
frequency signal and presenting said lower frequency signal at said
second terminal means and in the motor mode by chopping said lower
frequency signal at the frequency of said higher frequency signal
and presenting it at said first terminal means.
20. A power conversion system for converting between electrical
power at different frequencies, comprising:
a motor including a rotor, a stator, first and second independently
controllable field windings and at least one armature winding for
each phase;
a switching circuit including a plurality of switching devices and
having first terminal means interconnected with said armature
winding which carries a higher frequency signal established by said
machine and a second terminal means for interconnection with an
electrical power source establishing a lower frequency signal;
a first field control circuit for monitoring said machine to sense
phase difference between the optimum zero crossings and actual zero
crossings of said higher frequency signal for driving said first
field winding to adjust the phase of said higher frequency signal
to minimize said phase difference;
a second field control circuit for modulating said higher frequency
signal carried by said armature winding with said lower frequency
signal and for monitoring said second terminal means to sense
amplitude difference between a selected one of the voltage and
current parameters of said lower frequency signal and a reference
level for driving said second field winding to adjust that
parameter towards said reference level; and
a switch-firing circuit responsive to machine voltage and said
voltage and current parameters of said lower frequency signal at
said second terminal means for selectively triggering to the on
state and self-commutating to the off state said switching devices
synchronously with said zero crossings of said higher frequency
signal for transferring power between said higher and lower
frequency signals through said switching circuit by chopping said
lower frequency signal at the frequency of said higher frequency
signal and presenting it at the first terminal.
21. A power conversion system for converting between electrical
power at different frequencies, comprising:
a generator including a rotor, a stator, first and second
independently controllable field windings and at least one armature
winding for each phase;
a switching circuit including a plurality of switching devices and
having first terminal means interconnected with said armature
winding which carries a higher frequency signal established by said
machine and a second terminal means for interconnection with an
electrical load establishing a lower frequency signal;
a first field control circuit for monitoring said machine to sense
phase difference between the optimum zero crossings and actual zero
crossings of said higher frequency signal for driving said first
field winding to adjust the phase of said higher frequency signal
to minimize said phase difference;
a second field control circuit for modulating said higher frequency
signal carried by said armature winding with said lower frequency
signal and for monitoring said second terminal means to sense
amplitude difference between a selected one of the voltage and
current parameters of said lower frequency signal and a reference
level for driving said second field winding to adjust that
parameter towards said reference level; and
a switch-firing circuit responsive to machine voltage and said
voltage and current parameters of said lower frequency signal at
said second terminal means for selectively triggering to the on
state and self-commutating to the off state said switching devices
synchronously with said zero crossings of said higher frequency
signal for transferring power between said higher and lower
frequency signals through said switching circuit by detecting the
lower frequency signal from the higher frequency signal and
presenting it at said second terminal.
Description
FIELD OF INVENTION
This invention relates to a power conversion system for converting
between electrical power at different frequencies, and to a rotary
electrical machine for use therein.
BACKGROUND OF INVENTION
Investigation of power conversion systems applicable to high speed
and variable speed shafts have received much attention recently
because of their ability to generate power at higher speeds with
reduced weight and size. They are of particular interest due to
recent interest in their use with windmills and flywheels. In
flywheel storage systems the conversion system, and particularly
the motor generator machine, must be capable in one mode of
generating power at higher and variable frequencies and presenting
it at constant and conventional lower frequency and in another mode
of accepting power at the constant lower frequency and converting
it to a higher variable frequency to drive the flywheel. When used
with windmills and other prime power sources the system must be
capable of converting the generated higher varying frequency
electrical signals to constant lower frequency signals. See Report
R-960, Interim Report on Research Toward Improved Flywheel
Suspension and Energy Conversion Systems, by David Eisenhaure,
George Oberbeck, Stephen O'Dea, and William Stanton.
In general, the variable speed (frequency) power must be converted
to an existing fixed frequency for use. Many different combinations
of conventional power conversion systems have been applied to this
task, each with its own deficiencies. Typical systems have employed
multiple machine configurations, variable mechanical speed
reducers, A.C.- D.C.-A.C. converter-inverters, cycloconverters,
field modulated down converters and many other arrangements. Many
of these systems have suffered from large weight and size,
inefficiency due to poor waveform quality and form factor, or
unreliability due to complexity and switching losses.
SUMMARY OF INVENTION
It is therefore an object of this invention to provide an improved,
efficient, integrated power conversion system for converting
between electrical power at different frequencies, and an improved
extremely efficient rotary electrical machine for use therein.
It is a further object of this invention to provide such an
improved system capable of transferring power either from the lower
to the higher or the higher to the lower frequency whether the
frequencies are fixed or varying.
It is a further object of this invention to provide such an
improved system which reduces harmonic content of the waveforms and
the need for filtering.
It is a further object of this invention to provide such an
improved system which produces controlled voltage, frequency and
power output despite varying frequency input.
It is a further object of this invention to provide such an
improved system in which the rotary electrical machine is a single
unit.
It is a further object of this invention to provide such an
improved system which may be used as a stand-alone power supply or
may be coupled with a power line.
The invention results from the realization that an improved rotary
electrical machine, for utilization with switching circuitry of an
improved power conversion system, can be constructed using two
independently controllable fields which induce a back EMF in the
rotary machine compatible with self-commutating operation of the
switching circuitry, and further that the controllable fields can
also be used to control relative amplitudes and phases of the
output voltage and current.
The invention features a power conversion sytem for converting
electrical power at different frequencies. It includes a rotary
electrical machine including a rotor, a stator, first and second
independently controllable field windings, and at least one
armature winding for each phase. There is a switching circuit
including a plurality of switching devices and having first
terminal means interconnected with the armature winding which
carries a higher frequency signal established by the machine and a
second terminal means for interconnection with an impedance which
establishes a lower frequency signal. The lower frequency signal
can be a zero frequency or D.C. signal. A first field control
circuit monitors the machine to sense phase difference between the
optimum zero crossings and actual zero crossings of the higher
frequency signal for driving the first field winding to adjust the
phase of the higher frequency signal to minimize the phase
difference. A second field control circuit modulates the higher
frequency signal carried by the armature winding with the lower
frequency signal and monitors the second terminal means to sense
amplitude difference between the selected one of the voltage and
current parameters of the lower frequency signal and a reference
level for driving the second field winding to adjust that parameter
towards the reference level. A switch firing circuit responsive to
the machine and to the voltage and current parameter of the lower
frequency signal at the second terminal means selectively triggers
the on state and self-commutates to the off state the switching
devices synchronously with the zero crossings of the higher
frequency signal, for transferring power between the higher and
lower frequency through the switching circuit.
The invention also features a rotary electrical machine, such as a
motor generator, motor, or generator, having a rotor, a stator, an
armature, and dual independently controllable field windings in
spaced quadrature for selectively shaping the back EMF waveform of
the machine to control amplitude and phase relations between the
voltage and current outputs of the machine.
In a preferred embodiment the rotary machine may be a
motor-generator or a motor or a generator. If it is a
motor-generator capable of operating in a motor mode and a
generator mode, its rotor is adapted for mechanical load and/or
drive. The impedance is an electrical load and/or power source and
the power transfer through the switching circuit occurs in the
generator mode by detecting the lower frequency signal from the
modulated higher frequency signal and presenting the lower
frequency signal at the second terminal means, and in the motor
mode by chopping the lower frequency signal at the frequency of the
higher frequency signal and presenting it at the first terminal
means.
When the machine is a motor, the impedance is an electrical power
source and the power transfer through the switching circuit occurs
by the chopping of the lower frequency signal at the frequency of
the higher frequency signal and presenting it at the first terminal
means. When the machine is a generator, the impedance is an
electrical load and the power transfer through the switching
circuit occurs by detecting the lower frequency signal from the
higher frequency signal and presenting it at the second terminal
means.
When the impedance includes an electrical power source such as a
utility company power line, the second field control circuit
monitors the current parameter of the lower frequency signal at the
second terminal means and when the system is operating as a
stand-alone system, the second field control circuit monitors the
voltage parameter of the lower frequency signal at the second
terminal means.
DISCLOSURE OF PREFERRED EMBODIMENT
Other objects, features and advantages will occur from the
following description of preferred embodiments and the accompanying
drawings, in which:
FIG. 1 is a schematic block diagram of a power conversion system
according to one feature of this invention;
FIG. 2 is a schematic block diagram similar to FIG. 1 showing the
power conversion system according to this invention including a
motor generator and being coupled to an external power source;
FIG. 3 is a more detailed schematic diagram of the switching
circuit of FIG. 1;
FIG. 4 is a more detailed schematic diagram of the firing circuit
of FIG. 1;
FIG. 4A is a truth table showing the inputs and outputs for the
decoder of FIG. 4;
FIG. 5 is a schematic diagram of a circuit which may be used to
implement the decoder of FIG. 4;
FIG. 6 is a cross-sectional schematic diagram of a two phase
inductor motor-generator which may be used to implement the rotary
electrical machine of FIG. 1 according to a second feature of this
invention;
FIG. 6A is a table showing the combinative effect of the two fields
A and B at each of the eight poles of the motor-generator of FIG.
6;
FIG. 7 shows idealized waveforms for the voltage output
superimposed on the back EMF of the machine operating as a
generator and the current output;
FIG. 8 shows the voltage due to field A and the voltage due to
field B and the back EMF voltage resulting from the summation of
voltages due to A and B;
FIG. 9 shows the current output waveform for each of the phases in
a two-phase system;
FIG. 10 shows the voltage output superimposed on the back EMF
voltage and the current voltage waveforms for the case when the
machine is operated as a motor;
FIG. 11A shows the relationship of the back EMF voltage and the
output voltage for one phase of a two-phase system supplying an
alternating current load when the machine is operating in the
generator mode with unity power factor;
FIG. 11B is an illustration similar to FIG. 11A with a power factor
of zero;
FIG. 11C is an illustration similar to FIG. 11A in the motor mode
for a unity power factor;
FIG. 12 is a chart illustrating idealized waveforms in the case of
a zero frequency or D.C. lower frequency signal for a three-phase
system, similar to the wave shapes shown in FIGS. 7, 8, and 9;
FIGS. 13A, B, and C illustrate the relative condition of the
voltage output waveforms and the back EMF of the machine operating
at unity power factor and zero power factor as a generator, and as
a motor at unity power factor;
FIG. 14 is a cross-section diagram showing the one-phase portion of
a three-phase inductor generator similar to FIG. 6; and
FIG. 15 is a view similar to FIGS. 6 and 14 of a three-phase wound
rotor generator design in which the armature windings are on the
stator and the fields are on the rotor.
The power conversion system according to this invention may be
accomplished using a rotary electrical machine including a rotor, a
stator, first and second independently controlled field windings,
and at least one armature winding for each phase. There may be one
or more phases, the field windings may be either on the stator or
the rotor. The system includes a switching circuit including a
plurality of switching devices such as SCR's, and the switching
circuit includes a first terminal interconnected with the armature
winding which carries a higher frequency signal inherently
established by the machine operation and a second terminal for
interconnection with an impedance, either a load impedance or a
power source impedance, which establishes a lower frequency signal
at that terminal. The lower frequency signal may be zero frequency
or D.C., as well as A.C. There is a first field control circuit for
monitoring the machine voltage to sense the phase difference
between the optimum zero crossings and the actual zero crossings of
the higher frequency signal for driving the first field winding to
adjust the phase of the higher frequency signal and minimize that
phase difference. In effect, the phase difference between the
output voltage and output current of the machine is being monitored
in this way. The internal voltage of the machine which determines
the optimum zero crossings may be monitored using an encoder which
indicates the relative position of the rotor and stator. The actual
zero crossings indicative of the output current zero crossings may
be determined by a polarity indicator. Any phase difference between
the two generates an error signal which is fed to the first field
which drives the machine to correct that error.
There is a second field control circuit for modulating the higher
frequency signal carried by the armature winding with the lower
frequency signal, and for monitoring the second terminal of the
switching circuit to sense the amplitude difference between the
selected one of the voltage and current parameters of the lower
frequency signal and a reference level, for driving the second
field winding to adjust that parameter towards the reference level.
If the second terminal means of the switching circuit is coupled to
an external power source, then it is the current parameter that is
sensed, and the reference is a current level. The two are combined
in a comparator and an error signal is used to drive the second
field to adjust the amplitude and phase of the output current
relative to the output voltage.
The rotary electrical machine may be a motor-generator if
bi-directional power conversion is desired, or may be simply a
motor or simply a generator. If it is a motor-generator, the shaft
of the motor-generator is typically connected to a device such as a
flywheel, which may either provide mechanical force to drive the
generator in the generator mode or be driven by the motor in the
motor mode. In such a case, the switching circuit would be
connected to an impedance which would operate in the motor mode as
a power source and in the generator mode as a load. Similar
arrangements would adhere in the motor and in the generator modes.
When the system is operating in the stand-alone mode, that is, it
is not coupled to an external power source, the comparator in the
second field control circuit monitors the voltage parameter on the
output from the switching circuit, and the reference parameter is a
reference voltage.
Report R-960, Interim Report on Research Toward Improved Flywheel
Suspension and Energy Conversion Systems, by David Eisenhaure,
George Oberbeck, Stephen O'Dea, and William Stanton, is
incorporated here by reference.
There is shown in FIG. 1 a power conversion system 10 according to
this invention including a rotary electrical machine 12 having a
first field, field A 14, and a second field, field B 16. Field A is
fed by field A control circuit 18, and field B by field B control
circuit 20. Switching circuit 22 having a first terminal 24
connected to the output of rotary machine 12 and a second terminal
26 connected to the output line 128, chops the lower frequency
signal at terminal 26 at a frequency equal to the higher frequency
provided in machine 12 in the motor mode, and detects the lower
frequency envelope of the higher frequency signal produced in
machine 12 in the generator mode. The switching circuit is
controlled by firing circuit 28, which synchronously switches
circuit 22 in a sequence dependent upon the mode of operation of
machine 12, motor or generator, and the optimum zero crossover
point of the voltage output from machine 12.
Field control circuit 18 includes a polarity indicator 32 which
provides a signal each time the higher frequency signal at terminal
24 crosses zero, i.e. the actual time that the output current
crosses zero, and a rotor/stator position encoder 34 which, by
reference to the position of the rotor relative to the stator,
provides a signal indicating the optimum zero crossover time, or
the time that the voltage crosses zero. The outputs of polarity
indicator 32 and encoder 34 are submitted to a phase-sensitive
detector 36, which provides an error to amplifier 38 if there is a
phase difference detected. Amplifier 38, in response to an error
signal, drives field A to adjust the phase of the output current
relative to the output voltage in terminal 24 and eliminate the
error. Field control circuit 20 includes a comparator 40 which
senses either the voltage or the current parameter on output line
128 and compares it with a like reference parameter, voltage or
current respectively, to provide an error signal if there is a
difference between the two parameters. That error signal is
provided to amplifier 42, which responds by driving field B 16 to
adjust the amplitude and phase relationship of the output current
and voltage to minimize that error. If output line 128 is coupled
to an external power source, then the relevant parameter is
current.
Throughout the specification, similar parts have been given similar
numbers accompanied by a lower case letter.
Alternatively, as shown in FIG. 2, rotary electrical machine 12a
may be a motor-generator having a flywheel 44 connected to a shaft.
System 10a is coupled with an external power source, such as power
company 46, which together with system 10a supplies user load 48,
such as a residence. Such a system may be used to maintain constant
loading on power company 46, so that during peak periods system 10a
operates as a generator to take energy from flywheel 44 and supply
power to user load 48 in conjunction with that supplied by power
company 46, while during low power drain periods, when user load 48
is below that constant set for power company 46, the extra power is
supplied through switching circuit 22a to motor-generator 12a
operating as a motor, which drives flywheel 44 to store energy
therein. When it is desirable to have a varying reference to
comparator 40 in field control circuit 20a, a load levelling
programmer 50, which varies the reference level over a period of
time, may be used. Load levelling programmer 50 may be a fixed,
cyclical program or it may respond to real time inputs, such as the
drain on the power company 46 output, as indicated by dashed line
52. In addition to these applications a prime mover 54, such as a
windmill, may be used to drive flywheel 44 and motor-generator 12a
in the generator mode. System 10a is not restricted to stationary
uses, as it may be used to power vehicles as well. In a vehicle,
flywheel 44 may be used to drive motor-generator 12a as a
generator, to drive another motor-generator set which drives the
wheels of an automobile and, during downhill runs when the
automobile is coasting, the second motor-generator set may be used
to supply power to switching circuit 22a to drive motor-generator
12a as a motor and put energy into flywheel 44.
Switching circuit 22 may include a plurality of switching devices
SCR's 1-8, circuit 22c, FIG. 3. SCR's 1-8 are connected in pairs,
in parallel, and oppositely polarized. SCR's 1 and 2 are connected
directly between terminals 24 and 26. Terminals 24 and 26 may
include two busses 24c and 24cc and 26c and 26cc, respectively. The
first pair of SCR's 1 and 2 are connected between terminals 24c and
26c; SCR's 7 and 8 are connected between terminals 24cc and 26cc;
SCR's 3 and 4 are cross-connected from terminal 24cc to terminal
26c; and SCR's 5 and 6 are connected from terminals 24c to 26cc.
There may be one such group of SCR's or other semiconductor devices
for each output phase of the system.
Switching circuit 22 is fired in a synchronous pattern with the
high-frequency signal from rotary machine 12 by means of firing
circuit 28a, FIG. 4, which includes a one of eight decoder 60
having eight outputs labelled 0 through 7, and having three inputs:
one from exclusive OR gate 62 and one from polarity sensor 64, and
one from rotor/stator position encoder 34. The inputs to exclusive
OR gate 62 are from polarity sensors 64 and 66. Polarity sensor 64
senses the output voltage V.sub.O on line 28, while polarity sensor
66 senses the polarity of output current I.sub.O on line 128,
through lines 68 and 70 respectively. When the polarities of the
output voltage and current are different, exclusive OR gate 62
indicates operation in the motor mode, and when they are the same,
in the generator mode. The eight outputs labelled 0-7 of decoder 60
are fed through OR gates 70, 72, 74, and 76, which respectively
operate SCR switches 1 and 8; 4 and 5; 2 and 7; and 3 and 6; FIG.
3. A truth table 80 for decoder 60 is shown in FIG. 4A. A 1
indicates motor operation and 0 indicates generator operation in
the motor-generator column. In the V.sub.O column, 1 indicates
positive output voltage and 0 negative output voltage; and in the
rotor column, 1 represents that the rotor position indicates
positive machine voltage and a 0 negative machine voltage. Thus in
the motor mode, when V.sub.O is positive, first SCR'1 and 8 will be
fired when the back EMF is positive, and SCR's 3 and 6 will be
fired when the back EMF is negative. In the generator mode, SCR's 2
and 7 will be fired for positive back EMF, and then SCR's 4 and 5
will be fired for negative back EMF. For negative V.sub.O, in the
generator mode SCR's 3 and 6 are fired for positive back EMF and
SCR's 1 and 8 for negative back EMF. In the motor mode for negative
V.sub.O, SCR's 4 and 5 are fired for positive back EMF and SCR's 7
and 2 for negative back EMF. In each of the two modes, motor and
generator, the sequence repeats itself. Decoder 60 may be
implemented by a group of AND gates 82, 84, 86, 88, 90, 92, 94, and
96, FIG. 5 with inverters 99.
The rotary electrical machine 12, and more specifically a
motor-generator 12a, may be implemented by a two-phase
field-modulated inductor motor-generator 100, FIG. 6, and includes
an inductor rotor 101 surrounded by an eight tooth stator including
stator teeth 102, 104, 106, 108, 110, 112, 114, and 116. The stator
structure is omitted for clarity. Armature or input-output winding
118 is wound on the stator and has access through two terminals
120. Field A and field B are also wound on the stator, windings
121, 123 respectively, and have access to their terminals 122 and
124 respectively. The structure shown in FIG. 6 represents but one
phase of the two-phase motor-generator. The second phase is simply
a duplicate of the first, axially stacked with the portion shown
and with the teeth 102-116 shifted to provide a second phase at
90.degree. with respect to the first phase. The field A winding 121
makes teeth 102, 104, 106, and 108 assume one magnetic polarity,
and teeth 110, 112, 114 and 116 assume the opposite polarity. Field
B winding 123 for the generator mode causes teeth 102, 106, 110,
and 114 to add flux in the same direction as field A and to oppose
field A in teeth 104, 108, 112 and 116.
The chart in FIG. 6A shows the combined effect of fields A and B on
the stator teeth.
In the generator mode, when the lower frequency is zero or D.C. at
terminal 26 and output line 28, the idealized waveforms for one
phase of a two-phase system are as shown in FIG. 7, assuming that
the machine has an inductive output impedance, which is normally
the case. V.sub.O 130, FIG. 7, is a square wave, and the generator
output, or the back EMF of machine 12, V.sub.E, has a step shape as
shown at 132 with V.sub.O 130 superimposed on it. V.sub.E 132 is
formed by the combination of the voltage due to field A, V.sub.A
134, FIG. 8, the voltage due to field B, V.sub.B 136, FIG. 8. The
combination of these two waveforms results in V.sub.E 132, shown in
full lines in FIG. 8 and in dashed lines in FIG. 7, superimposed on
V.sub.O 130. It is the control over the relative amplitudes of
these two voltages, V.sub.A and V.sub.B, which enable this
motor-generator to vary the phase relationship and the amplitude
relationship of the voltages and currents.
V.sub.E 132, in the form shown in FIGS. 7 and 8, produces an output
current, I.sub.O 138, in FIG. 7. This I.sub.O is actually I.sub.O1
or the output current for phase 1. The same wave shapes shifted
90.degree. apply to the second phase of the system, and the two
output currents I.sub.O1 138 and I.sub.O2 140 are shown in solid
lines after rectification in FIG. 9. The sum of the rectified solid
line waveforms 138 and 140 closely approximates the desired zero
frequency or D.C. output. The portion of V.sub.E which induces the
current 138 is that voltage difference 133 between the salient
portion 135 of V.sub.E 132 and the top of V.sub.O.
When the system is operated as a motor, FIG. 10, V.sub.O 130a
appears the same as V.sub.O 130, but each portion of V.sub.E 132a
appears as a mirror image of the similar portion of V.sub.E 132,
due to the fact that in the motor operation the current is moving
in the opposite direction, or is 180.degree. out of phase with
respect to that same current in the generator mode. Thus the output
current I.sub.O1 138a, FIG. 10, is similarly shifted by 180.degree.
with respect to the output current I.sub.01 138, FIG. 7.
The dual field system controls the phase relationship of the output
current I.sub.O1 with respect to the machine back EMF V.sub.E. If,
for example, the field voltage V.sub.A, FIG. 8, should increase
relative to V.sub.O, FIG. 7, the portion 140 of V.sub.E 132 would
increase, causing a resulting increase in the positive slope 142 of
the associated portion of I.sub.O 138, FIG. 7. The negative slope
144 of current I.sub.O would not be as steep, due to raised level
141, and therefore not reach zero before the voltage V.sub.E
switched from positive to negative. However, at that point the
slope would become much steeper as at 146, and would soon after
pass through zero. Because of this occurrence, there would be a
virtual phase shift between I.sub.O and V.sub.E. This would be
detected by field control circuit 13, FIG. 1, by means of
phase-sensitive detector 36, which compares the phase of I.sub.O
from polarity indicator 32 and that of V.sub.E from encoder 34. The
resulting error output from phase-sensitive detector 36 drives
amplifier 38 to cause field A 14 to decrease the field voltage and
thereby decrease voltage level V.sub.A so that levels 140 and 141
are symmetrically disposed about the output voltage V.sub.O, FIG.
7, and thus restore I.sub.O waveform 138 to its normal form.
With an alternating current load or power source at terminal 26,
line 28, FIG. 1, V.sub.O appears as sine wave 130b and V.sub.E, the
back EMF of the machine, appears as waveforms 132b. With a
two-phase system supplying an A.C. load instead of a D.C. load, the
wave shapes for varying power factors are shown in FIGS. 11A, B,
and C. In FIG. 11A, output voltage V.sub.O 130b, is sinusoidal,
while the back EMF of the machine V.sub.E 132b, is similar to
V.sub.O 132 for the direct current output, FIG. 7, but varying in
amplitude. This is so because as V.sub.O on line 28 varies, so too
does the input to comparator 40, amplifier 42. Thus as V.sub.O
increases, so too does the field voltage V.sub.E. This causes the
salient portions 135 of V.sub.E 132b, FIG. 11A, to increase,
thereby increasing the voltage difference 133a, which causes the
current to increase, so that the current I.sub.O follows the
voltage V.sub.O, ideally exactly, in FIG. 11A, since the system is
operating at a unity power factor.
If, however, the power factor is zero as shown in FIG. 11B, the
voltage difference 133b becomes minimum at the higher values of
voltage and maximum at the minimum values of voltage, so that the
current is out of phase by 90.degree. with the voltage.
With reference to FIG. 2, if the power factor is to be controlled,
the phase of the voltage V.sub.O and current I.sub.P, sensed on
line 52, are compared in phase-sensitive detector 51. If a phase
difference is detected an error signal is generated to phase shift
the current reference supplied to control circuit 20 via load
levelling programmer 50, so that the modulation of field B 16
adjusts the voltage differences 133 and thereby shifts the current
I.sub.O toward the desired phase relationship or power factor with
respect to the voltage V.sub.O.
In the motor mode, FIG. 11C, the wave shapes are similar but those
of V.sub.E are reversed.
In a three-phase system, the waveforms for one phase with a zero
frequency of D.C. load are shown in FIG. 12. There, V.sub.A FIG.
12, appears generally the same as V.sub.B, and has a more
symmetrical shape, and V.sub.E has three steps instead of two
relative to counterpart waveforms in FIGS. 7-10. The resulting
output currents, I.sub.O1, I.sub.O2, and I.sub.O3, the three
different stator/armature currents from the three different phases,
have truncated triangular waveforms which are phased at 120.degree.
to one another and more nearly approximate uniform D.C. output per
phase than the two-phase system.
FIGS. 13A and B depict V.sub.O and V.sub.E for one phase of a
three-phase system during an A.C. load for unity and zero power
factors in the generator mode, and FIG. 13C depicts them for unity
power factor in the motor mode.
The rotary electrical machine 12 according to this invention may be
structured as shown in FIG. 14, where one phase of a three-phase
generator 100a is shown, and the second and third phases are
axially stacked with the one shown. Machine 100a includes twelve
poles 202-226, in contrast to the eight poles of machine 100, FIG.
6. Winding 120a of field A is wound so that stator teeth 208, 210,
212, 214, 216, 218 are baised with one magnetic polarity, and
stator teeth 220, 222, 224, 202, 204, and 206 with the opposite
magnetic polarity. Winding 123a of field B, for the generator mode,
makes the flux in 206, 212, 218 and 224 aid field A, and the flux
in teeth 202, 208, 214, and 220 oppose field A.
Machine 12 may also be constructed as a wound rotor-generator
design 100b, FIG. 15, in which both the field A winding 250 and
field B winding 252 are wound in slots on rotor 254 to interact
with armature windings 256, 258, and 260, accessible through
terminals 262, 264, and 266, respectively, located on stator 270.
With this wound rotor design, all three windings for a three-phase
output may be provided in the same stator portion. Rotor 254
includes six poles 280, 282, 284, 286, 288, and 290, in which the
fields A and B combine to produce a resulting field +A; +A -B; -A
-B; -A; -A +B; A +B; respectively.
Although a specific switching circuit, firing circuit and field
control circuits have been shown in the embodiment of the power
conversion system for use with the dual field rotary machine, this
is not a necessary limitation of the machine invention. The dual
field machine is useful in other applications and with other
switching and control circuits. In addition, the specific
techniques for determining amplitude, phase, zero crossovers,
polarity and other control criteria are illustrative only and not
limitations on either the dual-field machine feature of the
invention or the power conversion system feature of this
invention.
Other embodiments will occur to those skilled in the art and are
within the following claims:
* * * * *